1. Field of the Invention
The field of the present invention relates to devices which are used to absorb energy in a structure or a machine by yielding over a long stroke.
As one of many examples consider forces imparted to transmission towers which carry power lines. Transmission towers are presently designed to withstand numerous types of loads which are imparted to them. A standard load is the dead load resulting from the weight of the transmission lines and the tower components. Other dead loads such as ice formation on the transmission tower and the lines may be encountered in some areas.
In addition to these standard loads, however, the transmission tower and its components are also subjected to random loads which are both transient and dynamic in nature. Wind is a serious problem, especially when accompanied by storms such as hurricanes and tornados. Wind imparts heavy vibration loads and other dynamic forces to the transmission tower. Random formation and shedding of ice on the conductor creates an unbalanced and dynamic ice load. Wind can combine with ice to increase the effect of each on loading of the transmission tower. Vandalism, aircraft impact, conductor breaks, stringing error, etc. also impart a significant dynamic load to the structure.
In addition to the problems mentioned above, internal structural failure of a tower or line will place a heavy load on adjacent transmission towers that are not broken and, therefore, are forced to carry the increased load. When a conductor, tower, or insulator is damaged at a certain location, this impacts all neighboring transmission towers which are suddenly forced to carry a greater load. In conjunction with a broken conductor, other components of the tower such as insulators, arms, guys and the tower itself may break. Such failures create a serious load problem on the remaining transmission towers.
2. Description of the Prior Art
Numerous devices have been used to reduce dynamic shock loads to structures by absorbing energy by plastic material deformation. Such devices often utilize either a rolling ring or torus which absorbs energy by cyclic plastic deformation when the supporting mandril is stroked. Other devices utilize simple bars which are directly pulled, twisted, or bent to achieve energy absorption by plastic material deformation. All of these devices require excessive length or kinematic linkages to achieve long-stroke energy absorption. No prior metallic devices have been compact enough to allow extension of ten to twenty times the initial length.
None of the prior art devices could be used to alleviate the serious problem of conductor break and attendant transmission tower failure which arises when one of the conductors or an adjoining part of the transmission tower breaks. The increased load on all neighboring towers cannot be easily withstood and the primary tower and the adjoining towers fail due to this increased load. A particularly dramatic occurrence is known as a "zipper effect" or "cascade failure." When the first conductor or tower fails, it places an undue load on the adjoining tower and it also fails and may even topple. This places an undue load on the next transmission tower which may also fail. This in turn places an undue load on the next tower and so on. The result is a multiple tower failure occurring in rapid succession. Instances have occurred where several miles of transmission towers have failed all due to the original failure of one conductor or tower.
One concept in prior art tower designs is to design energy dissipating elements into the tower components. Another prior art design is to reduce the incidence of cascading tower failures by including a stronger tower at short intervals in the line. The stronger tower is designed to remain intact under unbalanced longitudinal loads.
It has also been proposed to isolate supporting structures from extreme longitudinal loads by including load limiting attachment devices, that is, for example, "breakaway or swing" arms. While this approach will prevent failure of the supporting structures, it will not necessarily prevent extensive damage to the conductors and hardware. The load limiting devices have almost no energy dissipating capability, and there is a potential loss of great lengths of conductors.
Some prior art devices which utilize friction as an energy dissipation means have been incorporated into some conductor suspensions to alleviate the conductor break problem. However, such devices which utilize a slipping conductor shoe or clutch mechanism do not always slip or operate at the desired load level, due to variance in friction coefficient.
Therefore, although devices and tower designs have been produced in order to minimize transmission tower failure, once such a conductor break or failure occurs, the presently known tower designs and devices are unable to effectively handle the increased stress resulting therefrom. There is no known prior device which is compact enough to be simply incorporated in the conductor suspension and which is able to absorb these increased loads by long-stroke plastic material deformation to prevent the occurrence of multiple tower failures resulting from the initial failure of one conductor or one transmission tower.